The present invention relates to a multi-layer pipe as well as a method for its manufacture. Multi-layer pipes are preferably used when high demands exist against corrosion or abrasion.
Corrosion-resistant pressure vessels or pressure lines can be produced in a more cost-effective way than solid versions of corresponding materials when multi-layer pipes are used. This is achieved by load distribution on a thin, corrosion-resistant internal layer (e.g. stainless and acid-resistant steel) and a high-strength and pressure-proof external layer (e.g. fine-grained structural steel). Steel consumption can be considerably decreased as a result and a large part of the remaining steel consumption can be shifted to more cost-effective materials.
In certain grades, abrasion-resistant pipelines can only be manufactured when being executed as a multi-layer pipe (for instance with mechanical bonding, see below), since materials (e.g. high-strength steels with high hardness) can be used as an internal layer which for itself cannot be processed into pipes or only under great difficulties.
Other material combinations are possible in a great diversity but basically the combination possibilities of materials are restricted in this context only by the processing methods eligible in each case.
When creating the pipe sheathing, there are two possibilities                metallurgical bonding over the entire surface (requiring cladded plates as initial semi-finished product), and        merely mechanical bonding (for instance friction bonding) between internal and external pipe—preferably internal and external plates and their welding on the plate edges—.        
Manufacture of such multi-layer pipes is done as follows in Prior Art:
For multi-layer pipes with metallurgical bonding between the layers—for instance multi-layer pipes out of metal plates, preferably steel plates—a cladded composite plate made out of two different (steel) materials is used as an initial semi-finished product. The multi-layer pipe is then manufactured as follows:                At first the composite plate is produced by roll-bonding or explosion cladding,        then pipe forming is made in accordance with usual methods such as for example by means of a bending roller or a bending press and        subsequently welding occurs with the outer wall of the multi-layer pipe being executed in accordance with the usual pipe welding methods pursuant to the material used and inner wall welding occurring as deposition welding likewise pursuant to the material.        
The disadvantage of this procedure according to Prior Art is on the one hand the high cost of the initial semi-finished product and thus also of the final product, but on the other hand also insufficient availability of the initial semi-finished product, because production capacities are very restricted for it on a world-wide basis. Thus, as far as is known to the applicant and the inventor, only a few installations exist for the production of roll-bonded multi-layer plates, for instance in Austria and in Japan, but for example, not a single one in the Federal Republic of Germany. Neither installations for explosion cladding do hardly exist as far as is known to the inventor and the applicant. For example, at Dynamit Nobel at Burbach, Federal Republic of Germany, one of a few of such plants exists. The production engineering used for it is also a great problem and therefore expensive and intricate taking into consideration in addition that it is only available for very small production lots, anyhow.
Moreover, the number of materials, which can be processed in this way, is restricted. Thus, for example, certain abrasion-resistant steels cannot be used as an internal layer, if they can hardly be welded or not welded at all due to their high carbon content.
In the case of multi-layer pipes with mechanical bonding, several—preferably two—finished pipes are used as an initial semi-finished product. The process will be explained below by way of an example with two pipes (in the event of more layers, the explanations have to be understood accordingly):                two finished pipes are manufactured in close fit and moved into each other without friction with the external pipe requiring a higher yield point than the internal pipe        by expansion (mechanically—for example, by means of an expansion die—or by fluid pressure with the pipes placed into each other being pressed into a die comprising the external pipe) the internal pipe is pressed into the external pipe by elastic expansion of the external pipe. After the expansion forces are omitted, the external pipe places itself non-positively around the internal pipe due to the higher elastic resiliency        finally the two materials are welded on their faces.        
The disadvantage of this process of Prior Art is that the external pipe must have a higher yield point than the internal pipe, since otherwise the elastic resiliency of the external pipe causing the non-positive connection with the internal pipe and therefore being necessary, is missing. This is particularly disadvantageous, because high-strength materials—for instance, especially high-strength steels—as they are especially advantageous preferably for abrasion-resistant pipelines inside the pipe, have high or even very high yield points, and are therefore unsuitable for this manufacturing process.